The present invention relates to a method and unit for sterilizing packaging sheet material for manufacturing sealed packages of pourable food products.
As is known, numerous pourable food products, such as fruit juice, UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages formed, on fully automatic packaging machines, from packaging sheet material, which may be constituted by pre-cut blanks or by a continuous web or strip of packaging material which is folded and longitudinally sealed to define a continuous, longitudinally sealed packaging material tube.
The packaging material may be constituted by a single or multi-layer plastics material, polymeric material, mineral-filled polymeric material or a laminated paperboard-type material. By way of example, one known type of paperboard-type packaging material has a multilayer structure comprising a layer of paper material covered on both sides with layers of heat-seal material, e.g. polyethylene. In the case of aseptic packages for long-storage products such as UHT milk, the packaging sheet material comprises a layer of barrier material defined, for example, by an aluminum film, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material which eventually defines the inner face of the package contacting the food product.
For producing aseptic packages in conventional form, fill and seal-type packaging machines, the strip of packaging material is provided, e.g. unwound off a reel, and fed through a sterilizing unit in which it is sterilized, for example, by immersion in a bath of liquid sterilizing agent such as a concentrated solution of hydrogen peroxide and water.
More specifically, the sterilizing unit comprises a bath filled, in use, with the sterilizing agent in which the strip is fed. The bath conveniently comprises two parallel vertical branches connected at the bottom to define a U-shaped path of a length depending on the traveling speed of the strip and such as to allow enough time to treat the packaging material. For effective, fairly fast treatment, so as to reduce the size of the sterilizing chamber, the sterilizing agent must be maintained at a high temperature of, say, roughly 70xc2x0 C.
The sterilizing unit also comprises an aseptic chamber in which the strip of packaging material issuing from the sterilizing bath is treated mechanically (e.g. by means of drying rollers) and thermofluidically (e.g. by means of hot-air jets) to remove any residual sterilizing agent. The amount of residual sterilizing agent allowed in the packaged product, in fact, is governed by strict standards (the maximum permissible amount being in the order of a 0.5 parts per million). The aseptic chamber must also be maintained slightly above ambient pressure to ensure any leakage through the seals occurs outwards as opposed to inwards of the chamber, to keep out any contaminating agents.
Before leaving the aseptic chamber, the strip is folded into a cylinder and sealed longitudinally to form in known manner a continuous, vertical, longitudinally sealed tube. The tube of packaging material, in fact, forms an extension of the aseptic chamber and is filled continuously with the pourable product and then fed to a forming unit for forming individual packages and by which the tube is gripped between pairs of jaws to seal the tube transversely and form aseptic pillow packs. The pillow packs are separated by cutting the sealed portions between the packs, and are then fed to a final folding station where they are folded mechanically into the finished form.
Alternatively, the packaging material is sterilized by applying, on the side of the packaging material eventually defining the inner face of the package, a thin film of hydrogen peroxide, which is later removed by heating. Sterilizing units are also known in which the hydrogen peroxide is applied to the surface of the packaging sheet material by liquid atomization or gas condensation.
Packaging machines of the above type are used widely and satisfactorily in a wide range of food industries, and performance of the sterilizing unit, in particular, is such as to amply conform with standards governing asepticity of the packages and residual sterilizing agent.
Within the industry, however, demand for further improvement exists, especially with regard to elimination of residual sterilizing agents, and which stems, in particular, from market demand for packages featuring reclosable opening devices which are easy to open and provide for easy pouring of the product.
In the case of non-aseptic packaging machines, such devices can be applied (e.g. injection molded) directly to the strip material before the packages are formed. Conversely, in the case of aseptic packaging machines, opening devices are normally applied after the packages are formed, which poses drawbacks from the production standpoint by requiring the use of sophisticated systems for supplying and applying the devices.
The above mentioned opening devices, if applied beforehand to the packaging sheet material, form breaks in the geometric continuity of the packaging sheet material, in which residual sterilizing agent may become trapped, and from which the sterilizing agent cannot be removed completely using known techniques.
On the other hand, using additional means to remove the sterilizing agent may have a negative effect on the operating parameters, in particular temperature and pressure, of the aseptic chamber, and impair performance of the sterilizing unit as a whole.
To eliminate the above drawbacks, sterilizing units have been devised using electron beams to irradiate moving sheet packaging material. More specifically, as known from SE-A-9503810, the sheet packaging material is irradiated on one face by an electron beam issuing from an accelerator located to one side of the packaging material. The electron beam penetrates the packaging material and so sterilizes both opposite faces simultaneously.
The above method, however, is not without drawbacks of its own, which mainly lie in alteration of the packaging material by passage of the electrons. More specifically, the packed product can acquire, from the packaging material, an unpleasant, so-called xe2x80x9coff-flavorxe2x80x9d.
It is an object of the present invention to provide a method of sterilizing sheet packaging material for producing sealed packages of pourable food products, designed to provide an effective, straightforward, low-cost solution to the aforementioned problems.
According to the present invention, there is provided a method of sterilizing sheet packaging material for producing sealed packages of pourable food products, the method comprising the step of irradiating the packaging material with electrons, wherein the irradiating step comprises the step of directing on to opposite faces of the sheet packaging material respective low-voltage electron beams, each having an energy of at most 100 keV.
The present invention also relates to a unit for sterilizing sheet packaging material for producing sealed packages of pourable food products, the unit comprising generating devices for generating low-voltage electron beams, which are located on opposite sides of the sheet packaging material and generate respective electron beams directed on to opposite faces of the packaging material and each having an energy of at most 100 keV.